Deprotonation of a formato ligand by a: Cis-coordinated carbyne ligand within a bis(phenolate) tungsten complex

Article abstract of DOI:10.1039/c8dt03056a

Deprotonation of a formato ligand by a cis-coordinated propylidyne ligand in a tungsten(vi) complex [(OSSO)W(CEt)(OCHO)] (3) that contains a tetradentate bis(phenolato) ligand (OSSO = {1,4-dithiabutanediyl-2,2′-bis(4,6-di-tert-butyl-phenolato)}) gave the dioxo complex [(OSSO)WO₂] (4) along with CO, ethylene and propylene as major products of decomposition.

Full text of DOI:10.1039/c8dt03056a

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Deprotonation of a formato ligand by a
cis-coordinated carbyne ligand within a
bis(phenolate) tungsten complex†
Cite this: DOI: 10.1039/c8dt03056a
Received 26th July 2018,
Accepted 28th August 2018
a
a
b
a
DOI: 10.1039/c8dt03056a
Tobias Schindler, Albert Paparo, ‡ Haruka Nishiyama, Thomas P. Spaniol,
b
b
a
Hayato Tsurugi,
Kazushi Mashima * and Jun Okuda
*
rsc.li/dalton
Deprotonation of a formato ligand by a cis-coordinated propyl- 1a containing a tetradentate OSSO-type ligand (OSSO =
idyne ligand in a tungsten(VI) complex [(OSSO)W(uCEt)(OCHO)] (3) {1,4-dithiabutanediyl-2,2′-bis(4,6-di-tert-butyl-phenolato)}) and
1
4
that contains
1,4-dithiabutanediyl-2,2’-bis(4,6-di-tert-butyl-phenolato)}) gave this ligand framework to study the interaction between a
the dioxo complex [(OSSO)WO ] (4) along with CO, ethylene and formato ligand and a cis-coordinated alkylidyne ligand.
a
tetradentate bis(phenolato) ligand (OSSO
=
the alkylidene complex 1b (Scheme 1). We have now used
{
2
1
4
propylene as major products of decomposition.
Salt metathesis of [(OSSO)W(uCEt)Cl] (2) and sodium
formate NaOCHO gave the formate propylidyne complex
Formic acid and metal formates generally decompose either
[
(
(OSSO)W(uCEt)(OCHO)] (3) as dark orange crystals
Scheme 2). The H NMR spectrum of 3 in C D indicates a
6 6
(a) via decarboxylation producing carbon dioxide and dihydro-
1
gen or (b) via decarbonylation to give carbon monoxide and
C
1
-symmetry in solution with two doublets of quartets at
unit of the propylidyne and a
1
,2
water. Thermally induced decarboxylation of metal formate
δ 4.47 ppm due to the CH
2
3
complexes leads to formation of hydride complexes. The
singlet at δ 9.35 ppm for the CH fragment of formate. The
reverse insertion of carbon dioxide into metal-hydride bonds
latter resonance is downfield shifted compared to that in other
4
produces metal formates. Upon deprotonation, formato
4,7
13
1
terminal formate complexes. The resonances in the C{ H}
NMR spectrum at δ 303.6 ppm for the alkylidyne carbon and
at δ 172.3 ppm for the formato carbon are consistent with pre-
ligands eliminate carbon monoxide to give terminal oxo
5
,6
complexes. Following recent work on stabilizing the dianion
of carbon dioxide (“carbonite”) through deprotonation of a
formate complex, we became interested in the intramolecular
deprotonation of a formato ligand within a Schrock-type
carbyne complex, since the basicity of the alkylidyne ligand is
viously reported values in other alkylidyne and formate com-
4,7,14
plexes.
The IR spectrum of 3 shows a strong asymmetric
−1
stretching vibrational mode νasym(CvO) at 1706 cm . Single
crystals suitable for X-ray diffraction were grown from
THF/n-hexane at −35 °C, but quickly decomposed during X-ray
diffraction study (ESI). The molecular structure shows an
4
,5,7–11
well documented.
Protonation of the negatively polar-
δ+
δ−
ized carbyne center M uC –R (M = Mo, W) was used to
deprotonate phenols, hydrogen halides and carboxylic acids,
1
η -formate with the formate proton turned toward the
1
2,13
giving alkylidenes.
Alkylidyne and alkylidene ligands can
be mutually interconverted through protonation and
1
2–14
deprotonation.
Alkylidyne complexes are sensitive to
hydrolysis: addition of water results in successive protonation
to the respective alkanes and terminal or bridging oxo metal
8
,15–18
complexes.
Recently we have demonstrated the reversible
interconversion between the high-valent tungsten alkylidyne
a
Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1,
5
2056 Aachen, Germany. E-mail: jun.okuda@ac.rwth-aachen.de
b
Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka,
560-8531, Japan. E-mail: mashima@chem.es.osaka-u.ac.jp
†
Electronic supplementary information (ESI) available. CCDC 1838111 (for 4)
and 1853496 (for 3). For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c8dt03056a
‡
Current address: School of Chemistry, PO Box 23, Monash University, VIC,
800, Australia.
Scheme 1 Reversible interconversion between alkylidyne and alkyli-
14
3
6 5 4
dene ligand (with [A] = [B(C F ) ]).
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Scheme 2 Synthesis of [(OSSO)W(uCEt)(OCHO)] (3) and isotopically
13
labelled C-3 and D-3 via salt metathesis of 2 with the respective
sodium formates.
propylidyne ligand and helical cis α-coordination of the
(OSSO)-ligand (Fig. S30†).
Fig. 1 Molecular structure of 4 in the solid state with displacement
parameters at the 50% probability level. Hydrogen atoms are omitted for
clarity. Selected interatomic distances (Å) and angles (°): W1–O1
To probe the interaction between the formato and propyli-
dyne ligand, isotopically labelled derivatives of complex 3 were
1.942(6), W1–O2 1.944(6), W1–O3 1.693(7), W1–O4 1.720(6),
1
3
1
prepared with NaO CHO and NaOCDO. The
H
NMR
exhibits a JCH coupling constant for
the O CHO unit of 214 Hz, similar to those in other
O1–W1–O2 149.1(3).
1
3
1
6 6
spectrum of C-3 in C D
1
3
1
3
7
C-labelled formate complexes. Isotopic labelling leads to a not incorporated into ethylene or propylene, both compounds
shift of the absorption frequency νasym (CvO) from 1706 cm−
1
must originate from the propylidyne moiety. Consumption of
(
3) to 1667 cm− ( C-3) or 1678 cm (D-3), which correspond the formate resonance at δ 172.3 ppm was accompanied by for-
1
13
−1
1
3
to a ratio of R(ν(3)/ν( C-3)) or R(ν(3)/ν(D-3)) of 1.02 due to a mation of carbon monoxide (δ 184.5 ppm) as well as other
7,19
larger reduced mass.
minor species such as butanal (δ 200.6 ppm) and 2-methyl-
2
2
Compound 3 slowly decomposed in solution to the dioxo propanal (δ 193.9 ppm). The formation of CO is explained by
complex [(OSSO)WO ] (4). However, 3 was more stable if all intramolecular deprotonation of the formato ligand, whereas
2
glassware was silylated with chlorotrimethylsilane before use. hydride abstraction should lead to decarboxylation.
The dark brown complex 4 was prepared by (a) adding water to
a solution of 1a or 3, (b) by heating a solution of 1a in C D
To determine whether the proton is transferred from the
formate unit to the alkylidyne ligand, we investigated the
thermal stability of D-3 in C D . Upon heating, the resonances
6
6
6
6
under air at 80 °C or (c) by slow decomposition of 3 in C
D
6
6
1
under an inert gas atmosphere at 60 °C. In the latter reaction, for the propylidyne ligand in the H NMR spectrum at
1
4
C
was obtained in 83% yield. The H NMR spectrum of 4 in δ 4.47 ppm (m) and δ 0.87 ppm (t) were consumed in favor of
indicates C symmetry in solution with a single set of ethylene and propylene in a ratio of 1 : <0.4. The alkene and
6
D
6
2
signals for the ortho- and para-tert-butyl substituents at vinylidene units of propylene at δ 5.72 ppm and at 5.01 ppm
δ 1.52 ppm and δ 1.17 ppm, similar to the dioxo molybdenum produce a weak resonance with altered coupling patterns,
2
0
analogue. Fig. 1 shows the molecular structure of 4 in the suggesting D-labelling. Most likely, the formato ligand in 3 is
crystal. The O1–W1–O2 angle is significantly bent from linear- deprotonated by the carbyne to give the alkylidene intermedi-
ity, indicating a distorted octahedral coordination geometry. ate Aa, which subsequently decarbonylates to an oxo-
The WvO bond lengths (1.693(7) Å, 1.720(6) Å) are similar to alkylidene species B (Scheme 3). Alternatively, the reaction
2
1
other hexavalent tungsten dioxo complexes.
may proceed via a concerted five-centered transition state,
We monitored the reactivity of 3 by H NMR spectroscopy directly eliminating CO and producing B (Scheme S1†). The
in C at 60 °C. Upon heating, the singlet at δ 9.35 ppm for latter eliminates ethylene or propylene while reacting with an
1
6 6
D
the formate proton and the resonances at δ 4.47 ppm (m) and unidentified source of oxygen to give 4. The interaction
3
δ 0.87 ppm (t, J = 7.5 Hz) for the ethyl group of the propyl- between the glass surface and the source of the second oxygen
HH
idyne ligand gradually decreased; ethylene and propylene were atom are not clear. A reversible interconversion between
2
2
formed with a ratio of 1 : 2 (Fig. S15†). In comparison to the alkylidyne and alkylidene ligands may be affected by a kinetic
low thermal stability of 3, solutions of the alkoxo complex 1a isotope effect and account for the lower ratio between propyl-
6 6
and the chloro complex 2 are stable when heated in C D at ene and ethylene for D-3 than for 3. The generation of butanal
1
3
13
13
6
{
0 °C. The intense C signal of the formate of C-3 in the
C
and 2-methylpropanal is explained by insertion of a CO frag-
1
H} NMR spectrum allowed tracing the fragments formed ment into the metal carbene-bond to eliminate the aldehydes
during decomposition. Since the C-labelled C
1
3
-fragment is and give 4 similar to a hydroformylation reaction (Scheme 3).23
1
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alkylidene Ab; subsequent hydrolysis of the formato or tert-
butoxido ligand generates formic acid or tert-butyl alcohol and
the oxo-alkylidene B which eventually decomposes to 4
(
Scheme 3).
In conclusion, we have shown that the high-valent tungsten(VI)
complex 3 containing a formato- and a cis-coordinated alkyli-
dyne ligand undergoes deprotonation and decarbonylation to
produce 4, CO, ethylene and propylene as major products of
decomposition. Both reactions, deprotonation of the formato
ligand and hydrolysis by water, result in the formation of
complex 4.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was funded by the German Research Foundation
through the International Research Training Group “Selectivity
in Chemo- and Biocatalysis”. A. P. gratefully acknowledges a
fellowship by the Fonds der Chemischen Industrie and T. S. a
scholarship by the Hans-Böckler-Stiftung. This paper is dedi-
Scheme 3 Proposed formation of 4 through intramolecular deproto-
nation of 3 to the oxo alkylidene (B) species, through hydrolysis of 1a−3
via the intermediate species Ab ([W] = (OSSO)W, [O] = unidentified
oxygen source, [H] = unidentified proton source, X = OCHO, OCDO, cated to the memory of Pascual Royo.
t
O Bu). The dominant pathway involves cleavage of ethylene or propy-
lene from B to produce 4.
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